Central heating expansion tank overflowing from vent pipe

[alice123]

Hi here i have seen several posting here for similar subject
My hot water cylinder split a few weeks ago, thankfully it was only a drip, however i decided this would be a good time to upgrade the system, i have a mistral oil fired boiler and a open vented C/H system, there has never been any discrimination to select just hot water or C/H only and the water never got very hot, but since fitting new cylinder and Drayton control unit with 3 way valve and cylinder stat the whole central heating system is great, But then i discovered that the bathroom radiator was on the HWC curcuit, this wasn’t a concern just more work. So now i have put the bathroom radiator on the proper circuit, which has now created another problem which is:
• The pump speed is set to no.2 and all the radiators heat up fine
• Select hot water and heating still all good
• Select just hot water and the water gets hot but a small amount of water passes from the vent line into the expansion tank in the roof. Turn the pump speed down to No.1 and this then stops, however on pump speed setting No.1 , two of the radiators struggle to get hot as per the other radiators
Do i just need to put some sort of restriction valve in HWC circuit, if so any idea what type

 

[misterdubya]

Sounds as though you have a partial blockage in the Heating System.

Are you able to post pics of your heating pipes layout in the airing cupboard?

Mr. W.

 

[alice123]

Hi Mr W
Thanks for responding, I do have a picture of pipe work to HWC but it’s of the old cylinder it does include the vent line from the HWC and fill line from expansion tank, it’s pretty much the same now but with a few more elbows i have also included picture of boiler pipe work.
This only became an issue after removing the bathroom radiator off the HWC circuit, the original boiler was a coal fired boiler which i replaced in 1998 shortly after moving in to the property, this was after opening the hopper door one morning to top the boiler up with coal and it spit out a nice flame in my face, the boiler was replaced a few days later.
Best regards

Geoff

Untitled

• alice123
• 26 Feb 2011
• 1

Untitled

• alice123
• 26 Feb 2011
• 1

 

[TicklyT]

If the cold feed and vent are in the same relative positions now, that could be contributing to the problem.

Older systems with gravity circulation were piped up with feed and vent either side of the boiler (or cylinder primary, whichever way you look at it). The pressure differential between feed and return was minimal with gravity circulation, and it provided an extra circulation path if anything untoward happened to a system heated by a relatively slow to respond solid fuel boiler, causing it to overheat.

Fully pumped systems run with a higher pressure differential between boiler feed and return. They are usually piped with the vent and cold feed close to each other, usually both connect to the pipe between the boiler and pump. this minimises the pressure differential between them.

The gravity circulation path is unnecessary if you have a boiler with more effecive cotrols than was possible with a solid fuel one.

 

[keego]

If it were my system I would be tempted to fit a bypass on the HW circuit just after the 3 port linking into the return by the boiler, this would then be doing the same job as the bathroom radiator.
Not the ideal / perfect solution but it should solve your problem of pumping over with minimal disruption.

 

[mysteryman]

You should sort this out with the cold feed and vent close-coupled as suggested, straight off the top of the boiler or in the flow to the pump. Don't run it for long in this state, because the rads and boiler will rot out very quickly.

 

[alice123]

Thank you all for responding, this makes sense now, i do like the idea of connecting the expansion tank feed & vent line between the boiler and pump but unfortunately this would involve a lot of work due to the distance between expansion tank and boiler, I think in the short term a bypass with a valve for balancing placed after the 3 way valve will be the easiest and cheapest option for now. I will let you know how i get on.
Once again thank you for the replies
Very best regards
Geoff

 

[bengasman]

A bypass may cure the symptoms, it will not solve the problem. Your system is almost surely heavily corroded, which needs to be corrected.